The UN flag at the organisation’s HQ in New York. Dozens of current and former employees described a culture of silence. Photograph: Alamy Stock Photo

The United Nations has allowed sexual harassment and assault to flourish in its offices around the world, with accusers ignored and perpetrators free to act with impunity, the Guardian has been told.

Dozens of current and former UN employeesdescribed a culture of silence across the organisation and a flawed grievance system that is stacked against victims.

Of the employees interviewed, 15 said they had experienced or reported sexual harassment or assault within the past five years. The alleged offences ranged from verbal harassment to rape.

Seven of the women had formally reported what happened, a route that campaigners say is rarely pursued by victims for fear of losing their job, or in the belief that no action will be taken.

“If you report it, your career is pretty much over, especially if you’re a consultant,” said one consultant, who alleged she was harassed by her supervisor while working for the World Food Programme. “It’s like an unsaid thing.”

The UN conceded that under-reporting is a concern but said the organisation’s secretary general, António Guterres, has “prioritised addressing sexual harassment and upholding the zero tolerance policy”.

Employees working in more than 10 countries spoke to the Guardian on condition of anonymity, partly because they are precluded from talking publicly by UN rules governing staff, partly for fear of retaliation.

Three women who reported sexual harassment or sexual assault, all from different offices, said they had since been forced out of their jobs or threatened with the termination of their contract in the past year. The alleged perpetrators, who include a senior UN official, remain in their posts.

In cases involving the exploitation of local people or occurring within the UN, complaints are difficult to pursue because of the organisation’s international nature. Many senior staff have diplomatic immunity, meaning they can avoid national courts.

“The culture of being a silent bystander is so pervasive at the UN, for reasons that don’t apply to Hollywood or the tech industry,” said Paula Donovan, co-director of Aids-Free World and the Code Blue campaign, which aims to end impunity for sexual abuse by UN peacekeepers. The sheer size of the UN – which employs roughly 44,000 staff – means perpetrators can easily be moved elsewhere.

Alex Haines, a barrister, said the UN’s internal justice system routinely fails to protect against glaring conflicts of interest. He cited a 2015 case that took place in central Asia, where a man accused of sexual harassment was allowed to interview the woman who brought the complaint against him. Such practices are not uncommon, he said, adding that victims are also prevented from reading the final report produced by investigators.